1)as, when 引导短暂性动作的动词。例如:
Just as / Just when / When I stopped my car, a man came up to me. 我一刹车,有一个人向我走来。
2)当从句的动作发生于主句动作之前,只能用when 引导这个从句,不可用as 或 while.例如:
When you have finished your work, you may have a rest. 赶完活后,你可以休息一下。
3)从句表示"随时间推移"连词能用as,不用when 或while.例如:
As the day went on, the weather got worse.日子一天天过去,天气越变越坏。